Ethanol is receiving attention as a fuel source substituting for an oil fuel. The market size of ethanol is predicted to be 55 million kilo litters in 2010. Adoption of methanol as a fuel, however, requires distillation of a crude product obtained from a biomass such as corn, and then dehydration of the resultant product to obtain at least 99.5 wt % or more.
What has conventionally practiced for such dehydration is to: distill and concentrate a dilute ethanol aqueous solution in a distillation tower until ethanol/water azeotropic point is nearly reached; and then dehydrate the distillate.
One of dehydration methods involves addition of an entrainer and dehydration through azeotropic distillation. This method, however, requires processes such as the azeotropic distillation of three-component system and recovery of the entrainer, and therefore has several drawbacks such as needing an enormous amount of heat energy.
In another one of dehydration methods, multiple molecular sieve vessels are arranged in parallel, and dehydration is performed by switching the molecular sieve vessels on a batch basis. This method too, however, has a drawback of consuming an enormous amount of energy to recover the molecular sieve vessels.
Then, it is conceivable to use an element, such as a separation membrane, that does not bring the above drawbacks (Patent Literature 1: JP 58-21629 A). A method using such separation membrane, however, is actually impractical because too much cost is required for a water separation membrane for large-scale dehydration.
[Patent Literature 1] JP58-21629 A